1. Field of the Invention
The invention relates to an information reader device provided to recognize letters and numbers denoted on a license plate for motor vehicles.
2. Description of Prior Art
In general, a vehicular license plate has a letter sequence indicative of the land transportation agency, a vehicular type classification number, a vehicular distinction letter and a license number. These letters and numbers are a main source to recognize and identify a motor vehicle.
It has been possible to read out the numbers and letters denoted in the license plate by the following procedures.
Detecting the position of the license platexe2x86x92extracting segments of the lettersxe2x86x92recognizing the letters.
In an automatic administration system for motor vehicles in which entrances and exits of motor vehicles are automatically managed at parking lots or toll gates on highways, it is an important technology to accurately extract the letter sequence section from an area in which the license plate occupies in order to ameliorate the literal recognition.
The following procedures have been known to extract the letter sequence section from an area in which the license plate occupies.
(i) The letter sequence is detected based on the information where the license plate is located as disclosed in Laid-open Japanese Patent Application No. 2-226479.
(ii) The license plate is transformed into an image which is then converted into two-value image. Size of connection area of the two-value image is measured to search the corresponding connection area of the letter sequence denoted on the license plate so as to recognize the letter sequence on the license plate as disclosed in Laid-open Japanese Patent Application No. 4-242493.
(iii) Based on the image transformed from the area of the license plate, a histogram is prepared to detect the letter sequence on the license plate.
However, the procedure (i) directly extracts the letter sequence on the license plate only from the information in which the license plate positions without detecting the information based on the letter sequence of the license plate. For this reason, it is feared that the extracted letter sequence would be significantly out of place unless accurately detecting the position of the license plate.
In the procedures (ii) and (iii), prior to treating with the two-value conversion, it is necessary to eliminate the noise caused from a mounting frame, bolts and the like used when securing the license plate to the motor vehicle.
Particularly for a low contrast image and stains on the license plate, it would be difficult to sufficiently distinguish the letter sequence on the license plate from the background of the license plate, thus resultantly deteriorating the accuracy when extracting the letter sequence on the license plate (one of the drawbacks when treating with the two-value conversion).
With the above drawbacks in mind, the present invention has made, it is a main object of the invention to provide a license plate information reader device for motor vehicles which is capable of remaining substantially immune to the peripheral noise caused from the mounting frame, bolts and the like used when securing the license plate to the vehicle, and substantially immune to the image noise caused by the stains and low contrast image on the license plate.
Therefore, it is another object of the invention to provide a license plate information reader device for motor vehicles which is capable of precisely extracting the letter sequence denoted on the license plate so as to attain the literal recognition with a high accuracy.
According to the present invention, there is provided a license plate information reader device for motor vehicles in which a front and rear portion of an incoming and outgoing motor vehicle is photographed by means of a photographing device so as to produce a video image data including a license plate. An A/D converter transforms the video image data into a digital image data. A license plate extracting device is provided to detect a position in which the license plate occupies so as to extract an image including the license plate. A literal region extracting device is provided to extract a literal positional region of a letter sequence denoted on the license plate based on the image obtained by the license plate extracting device.
Into a part of the literal region extracting device, a filter net is incorporated to serve as a neural network which has learned to emphasize a literal image of the literal positional region previously specified. This extracts only an area in which densely concentrated picture elements occupy among the extracted images including a plurality of image patterns serving as noise components.
With the literal region extracting device thus provided, it is possible to positively extract letters from the literal image of the literal positional region so as to facilitate the literal recognition.
With the filter net used to serve as the neural network in order to emphasize the literal image of the literal positional region, it is possible to remain substantially immune to the peripheral noise caused from the mounting frame, bolts and the like used when securing the license plate to the vehicle, and substantially immune to the image noise caused by the stains and low contrast image on the license plate. This makes it possible to precisely extract the letter sequence denoted on the license plate so as to attain the literal recognition with a high accuracy.
A literal recognition device is provided to recognize the letter from the literal image of the literal positional region obtained from the literal region extracting device. With the neural network used as the literal recognition device which produces a literal code corresponding to an input of the literal image extracted by the literal region extracting device, it is possible for the literal recognition device to produce a correct literal code.
With the filter net incorporated into a part of the literal region extracting device, the filter net learns the literal image including the license plate having a plurality or patterns and the literal regional area corresponding to each digit of the number denoted on the license plate. The filter net produces an output value in correspondence to each of the digits of the literal image of the license plate when the input image data is fed. The literal image of the literal image region is emphasized with a product of the input image data and the output value produced from the filter net.
With the first procedure putting an edge emphasis to a contour area of the obtained image, and further forming a compression image pattern in order to use it to a learning data for the filter net, and with the second procedure putting the edge emphasis to the contour area of the image obtained from the literal region extracting device, and forming a compression image pattern in order to use it for an input pattern fed to the filter net, it is possible to emphasize the literal image region of each of the digits of the literal image of the license plate.
In this instance, putting the edge emphasis makes it possible to extract common features from license plates of different colorations, and the compression image eliminates minor minutia unnecessary for the filter net to learn.